Fiber reinforced polymer (FRP) composites have surpassed their initial target applications in the aerospace industry to become a viable material alternative in sporting goods, automotive, and construction industries. High performance FRP composites made with synthetic fibers such as carbon, glass or aramid embedded in polymeric matrices provide the advantages of high stiffness and strength to weight ratio and increased chemical inertness compared to conventional construction materials, i.e., wood, clay, concrete and steel. In spite of these advantages, the widespread use of synthetic FRP composites has been limited, among several factors, due to their higher initial material costs, their use in non-efficient structural forms and their environmental impact.
Increased environmental awareness and the interest in long-term sustainability of construction materials have thus challenged the development of environmentally friendly alternatives to synthetic oil-based FRP composites (Mohanty et al. Macromol Mater Eng, Vol. 276/277, 1-24 (2000)). Natural-fiber-reinforced polymer composites, or bio-composites, have emerged in the past decade as an environmentally friendly and cost-effective option to synthetic FRP composites. Despite the interest and environmental appeal of bio-composites, their use has been limited to non-primary, or non-load-bearing applications due to their lower strength and stiffness compared with synthetic FRP composites (Biswas et al., “Development of Natural Fiber Composites in India”, Proceedings of the Composites Fabricators Association's Composites, Tampa, Fla. (2001)). Recent developments, however, have shown that the properties of “engineered” bio-composites (Mohanty et al., “Surface modifications of Natural Fibers and performance of the Resulting Bio-composites: An Overview,” Composite Interfaces, 8, 313-343, (2001); and Mishra, S., et al., Composite Science and Technology 63, 1377-1385, (2003)) are a technical, economical, and environmentally conscious alternative to E-glass fiber reinforced composites (the most common synthetic fiber composite) without sacrificing performance. While bio-composite materials with specific properties equivalent to entry-level structural materials are feasible, this performance level is still not enough to make them able to compete with existing construction materials. However, the structural performance of a component depends on both its material and structural properties. The lower material stiffness of bio-composites can thus be overcome by using efficient structural configurations that place the material in specific locations for highest structural efficiency. Natural fibers embedded in a natural or synthetic polymeric matrix, known as bio-composites, have gained recent interest because of their low material and manufacturing costs, light weight, high specific modulus (elastic modulus over density), and environmentally friendly appeal (Mohanty et al., Macromol Mater Eng, Vol. 276/277, 1-24 (2000)). Natural fibers are categorized depending on their source as either leaf or bast fibers. Bast fibers have the highest mechanical properties and thus are ones typically considered for structural applications. The most common bast fibers are flax, hemp, jute, and kenaf. Typical mechanical properties of these fibers together with E-glass fibers are given in Table 1. All natural fibers are lingo-cellulosic in nature with the basic components being cellulose and lignin. The density of natural fibers is about half that of E-glass (Table 1), which makes their specific strength quite comparable, while the elastic modulus and specific modulus is comparable or even superior to E-glass fibers. Hybrid bio-composite cellular structures can be used in multi applications (e.g., building walls, floors and roofs, bridge and ship decks, aircraft floors) with tailorable integrated multi-functions (i.e., stiffness, strength, thermal insulation, fire protection, and user friendliness). The sustainability and social acceptance of the proposed components, stemming from its large constituency on rapidly renewable resources, will pioneer the use of agricultural commodities in markets aimed at load-bearing materials and structures.
TABLE 1Mechanical Properties for Selected Natural Fibers and E-Glass FiberElasticTensileSpecificSpecificFiberDensityModulusStrengthModulusStrengthType(g/cm3)(GPa)(MPa)(GPa/g/cm3)(MPa/g/cm3)E-glass2.55732000-350029 780-1370Hemp1.487069047466Flax1.460-80 345-110043-57250-785Jute1.4610-30400-750 7-21275-510Sisal1.3338450-64029340-480
Pultrusion is a process of continuously forming reinforced plastic materials having a uniform cross-sectional profile. The word “pultrusion” is a hybrid which combines the words “pull” and “extrusion”. The product is literally pulled through a forming die. In its most usual form, pultrusion involves feeding a multiplicity of fiberglass roving strands, with or without additional plies of glass mat of appropriate width, into a pultrusion die. A resin, normally a thermosetting material such as polyester, is injected into the die where it is uniformly distributed among the reinforcing materials. Alternatively, the reinforcing material may be drawn through a resin bath prior to entry into the die. The die itself is heated. As the product is drawn from the die, the resin is either cured, or very nearly cured. The endless product so formed is then cut to appropriate length. Many variations of this general process have been developed as the technology has matured. Pultruded products are used in a great variety of applications. In many places they have replaced metallic construction materials, particularly those used in highly corrosive environments. Structural beams, floor gratings, handrails, ladders, and many similar products are now made by pultrusion process. A general background on pultrusion is found in an article by Martin, Modern Plastics Encyclopedia, pp. 40 317-318, McGraw-Hill, Inc., New York (1986).
U.S. Pat. No. 4,252,696 describes polyester resin compositions containing 4-10 parts of particular cellulose acetate butyrate resins per 100 parts of particular polyester resins which can be pultruded at greater speeds to give products having diminished surface roughness and internal and/or external cracking. These compositions have been found to give bulk or sheet molding compositions capable of producing thick moldings that are crack-free.
U.S. Pat. No. 4,541,884 describes pulling a continuous tow or roving of fibers through a mixture of a thermoplastic polymer and a volatile plasticizer. The plasticizer reduces the melt viscosity to achieve uniform impregnation of the reinforcing fibers. After forming the product, the plasticizer is volatilized. However, the reinforcing must have sufficient longitudinal strength to enable it to be drawn through the viscous impregnation bath. In this invention, at least 50% by volume of the fibers must be aligned in the direction of draw.
U.S. Pat. No. 4,028,477 describe a method for producing a pultruded product first by taking an open cell foamed core material and impregnating it with a thermosetting resin. The impregnated foam core is faced on one or both sides with a resin free fibrous reinforcing layer. The assembly is then molded in a pultrusion die where the resin flows from the foam into the reinforcement. The foam core is ultimately totally collapsed in the process. Cellulosic paper, cotton fabric, asbestos, nylon, and glass are disclosed as reinforcing materials.
Cellulosic materials have found very little use in any capacity in reinforced plastic materials based on polyester resins. They have had a long standing reputation, not without some justification, for causing soft cures and tacky surfaces. This has been particularly true for products based on wood fiber which have not been chemically modified. U.S. Pat. No. 3,361,690, describes the use of Douglassfirbark fiber as a reinforcing material for polyester-based bulk molding compounds. U.S. Pat. No. 3,248,467 describes the use of Douglassfirbark fiber as a reinforcing material in melamine overlaid reinforced plastic moldings. However, the bark fiber products appear to be an exception to the problems encountered with other cellulose based materials.
Purified cellulose has found widespread use in thermosetting resins such as impregnated phenolic and melamine laminates and molding compounds. However, it has not been generally regarded as useful in pultruded products. A few applications using helical wound paper have appeared in the patent literature. Japanese ‘patent application No. 56-17245 describes the use of a low density (ca. 0.7 g/cc or lower) paper tube which serves as a permanent mandrel for a pultruded cylindrical shape. The resin and glass composite surrounding the tube is bonded only to the surface and the tube itself is not impregnated with resin. It's apparent that the process is not simple in commercial scale and not cost effective.
Fr 2,391,067 teaches the use of a plurality of reinforcing fiber bundles, each of which is wrapped with a barrier material of paper. These may then be used in pultruded or extruded products. The barrier material serves to prevent passage of resin into the reinforcing fiber bundles. It is important to impregnate the barrier layer.
U.S. Pat. No. 3,470,051 describes reinforced plastic rods either as hollow tubes or solid tubes which includes an outer layer of longitudinally extending, exactly parallel, reinforcing glass fibers roving, impregnated with a resin-emulsion. The outer layer may be formed on a core and the layer may be produced simultaneously. A relatively complex helically wound preformed and then used it as a core for pultruded products such as arrow shafts. The preform has a double layer of paper, then a layer of glass, and finally another layer of paper. This is then coated with resin and molded into a rod before use in the pultrusion process. But it has been found critical to make higher width, high thick profile product.
An early article describing the pultrusion process (Machine Design. 43, Dec. 26, 1971, pp. 45-49) speculates that any material that can be fed from a coil is a “possibility” for use in the pultrusion process. Paper products, along with a host of other materials, are suggested as being potentially useful.
U.S. Pat. No. 4,983,453 describe a composite pultruded product and the method for its manufacture. The product is made with a plurality of longitudinal glass roving strands. In addition, a cellulosic mat is used in association with the roving. The mat serves as a filler, or reinforcing filler. In the method, cellulose-based material is completely resin saturated and then co-pultruded with a reinforcing glass rovings. It has been found critical to have desire strength with paper products. The process of making product by using only cellulose material and even in combination with glass rovings leads to non-uniformity in physical properties.
In none of these cases is synthetic non-woven fabrics encapsulated resin impregnated synthetic polyester felts has been used for making continuous high thickness profiles and plates.
Despite the enormous versatility of the pultrusion method and many variations which have been developed around it, it still has limitations. In many cases it is necessary to overdesign products in order to ensure uniform distribution of the glass reinforcement within the resin matrix. Products with low glass content tend to show areas of resin separation in which the reinforcing material may be completely absent. This resin separation is apt to occur even when high concentrations of mineral fillers are used with the resin. Pultruded products also tend to have relatively high density in comparison to many other plastic composites. This results in a relatively high cost per unit volume of the finished product. Further, because of the problem of resin separation at low reinforcing fiber contents, it is very difficult to make products less than about three millimeters in thickness, even though they may not be required to have high strengths. While some attempts have been made in the prior art to address these deficiencies, none have been particularly successful to the present time. Again product with high cellulosic fiber has limitation of pull strength and moisture content.
Accordingly, there is a long felt need to develop a simple yet technically improved and economically significant process of preparing composites, which is fast and simple, and yields improved products having uniform mechanical property, light weight still enhanced tensile and bending strength and high endurance subject to dynamic application.